Resonant optical transducers for in-situ gas detection

ABSTRACT

Configurations for in-situ gas detection are provided, and include miniaturized photonic devices, low-optical-loss, guided-wave structures and state-selective adsorption coatings. High quality factor semiconductor resonators have been demonstrated in different configurations, such as micro-disks, micro-rings, micro-toroids, and photonic crystals with the properties of very narrow NIR transmission bands and sensitivity up to 10 −9  (change in complex refractive index). The devices are therefore highly sensitive to changes in optical properties to the device parameters and can be tunable to the absorption of the chemical species of interest. Appropriate coatings applied to the device enhance state-specific molecular detection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/674,839, titled “Resonant Optical Transducers for In-Situ GasDetection,” filed Nov. 12, 2012, incorporated herein by reference, whichis a Continuation in Part of U.S. patent application Ser. No.12/406,838, titled “Tunable Photonic Cavities for In-Situ SpectroscopicTrace Gas Detection,” filed Mar. 18, 2009, incorporated herein byreference, which claims priority to U.S. Provisional No. 61/037,642,filed Mar. 18, 2008, titled: “Tunable Photonic Cavities for In-SituSpectroscopic Trace Gas Detection,” incorporated herein by reference andalso claims priority to U.S. Provisional No. 61/037,645, titled:“Resonant Optical Transducers for In-Situ Gas Detection,” filed Mar. 18,2008, incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to emission monitoring and control ofgases in various environments, and more specifically, it relates devicesfor in-situ gas detection.

Description of Related Art

Emission monitoring and control of gases in various environments iscritical in determining the overall health of a given system. Inparticular, low-weight molecule sensing (e.g., H₂, O₂, CO₂, NO₂, C₂H₂)will indicate possible inward/outward leaks (Ar, O₂, H₂O, He), identifydegradation (H₂O→corrosion, NO_(x)←He), identify aging mechanisms andincompatibilities, detect contamination and reactions (H₂, NO_(x)) anddetect outgassing (CO_(x), O₂). Such system designs call for in-situ,minimally invasive, trace-gas analysis, with the requirement of partsper million (ppm) sensitivity, at a few atmospheres pressure of N₂ airand within a cubic centimeter volume. In general, such sensors canbenefit with on-board, smart self-calibrating subsystems to augment thebasic device.

Examples of systems that can benefit from trace-gas sensors includecompact and highly multiplexed autonomous systems for laboratory orfield monitoring of nuclear, chemical or biological threats, forcombustion and environmental research, pollution monitoring, spaceexploration, aircraft cabin atmosphere monitoring, nuclear proliferationdetection, stockpile stewardship, etc.

Presently, there exist a variety of sensors based on optical techniques,including optoelectronics and photonics, optical fiber sensors, as wellas electrical and electromechanical technologies. Tables 1A and 1Btabulate prior art sensor techniques and properties as a function ofselected device parameters. Tables 1C and 1D provide similar devicecharacteristics, as applied to embodiments of the present invention. Foreach given sensor listed, entries in the table include its respectiveprinciples of operation, typical species to be sensed, performance,sensitivity, lifetime and other relevant device parameters, as well asrepresentative archival references.

Referring to tables 1A and 1B, to further place the present inventionwithin the context of state-of-the art sensors, are summarized relevantperformance figures-of-merit in this survey of existing and promisingtechnologies that have the potential to address typical applicationsissues, including the following: (1) low cross-sensitivity in thepresence of other trace gases; (2) time-dependent concentrationtracking; (3) limitations in terms of null outgassing; (4) stockpilestewardship; (5) relative compactness; (6) degree of maintenance; (7)moving parts, if required; (8) device weight; (9) power operation and(10) reliability and lifetime.

The prior art entries are partitioned in tables 1A and 1B, based on theunderlying technology and principles of operation, including thefollowing: (1) Specialty Optical Fibers; (2) Advanced CMOS ICs and (3)optoelectronic and photonic devices. The first set of entries includesexamples from the more mature and established class of sensors, such asNIR, mid-IR, and IR absorption techniques, with detection sensitivity inthe range of 10s of ppm, with very fast response times. Some examplesinclude NO₂ and CO, C₂H₂ detection using hollow fibers (see ref. i) andmicrobore fibers (see ref. ii), respectively. IR absorption studies ofCO₂ have been pursued using Fourier Transform IR techniques, FTIR (seeref. iii), and NO using Quantum Cascade (QC) lasers as optical probesources (see ref. iv). The main issues with these systems include thedevice dimensions, usually in excess of a cubic centimeter, and fiberocclusion, which leads to poor reliability.

Within the second class of basic trace-gas technologies, electrical andelectrochemical transducing techniques are included. In this class ofsensor, the measured parameter, in the presence of the desired tracegas, is typically a change in resistance, capacitance or current. Fullyintegrated systems with temperature and relative humidity controls arecurrently produced by Keibali Corp. and Sandia/H2SCAN for H₂ detectionusing Pd coated MOS capacitors, HEMT, or Schottky diodes withsensitivity down to 02% and milliseconds to seconds response times, witha few years of lifetime claimed (see ref. v). Other porous films ofmetal-oxides or metal-semiconductors with interdigitated electrodelayouts have been used to demonstrate NO₂, and CO_(x) detection belowthe ppm level (see ref. vi). The major application concern for thisclass of sensor is claimed to be the partial cross-sensitivity in thepresence of other gas species, which can be reduced using hightemperatures (thus the integration of micro-hotplates) and noble metals.Also, the response time of few minutes, dependent on the adsorptionrates, is higher relative to competing techniques such asgalvanic/electrochemical cell based sensors. Test cells, that arecommercially available at BW Technologies and RKI Instruments, indicatethe presence of a desired trace gas via changes in the measured current,as a result of ox-redox chemical reactions, with long lifetimes and widedynamic ranges of detection. Their versatility to detect several gases,e.g., O_(x), CO_(x), and NO_(x), becomes an issue since the level ofcross-sensitivity to different gases is very high. Porous Si FETs (seeref. vii) and thin films (see ref. viii) which, respectively, reveal achange in current and photoluminescence (PL), have a limitation in termsof saturation, or quenching. These sensors have been used for NOsensing, with sensitivities in the range of <1 ppm levels but the latterare limited by PL recovery time and a chronic PL quench.

Finally, in the third category tabulated in tables 1A and 1B, are listednew nanotechnology approaches such as SERS (Surface Enhanced RamanSpectroscopy), an example of which exploits substrates with silver (Ag)nanoclusters (˜100 μm²). These sensors have been currently used forcomplex molecules detection, e.g., H₂NO₃ (see ref. ix), HE, CBW agents(see ref. x) and are presently also being considered for PH and simplermolecular detection (see ref. xi). The limit of the SERS approachappears to be film oxidation, which limits the lifetime and thecomplexity of readout systems. At present, the level of sensitivityremains relatively high (100 ppm). Hydrogen sensing has also beendemonstrated by Zhao et al. (see ref. xii) using white-light reflectanceon palladium-gold (PdAu) thin films and by Villatoro/Olpiski et al. (seeref. xiii) using Pd nanotapered fibers at 1.5 μm signal wavelength.These novel techniques seem to be promising, having demonstratedacceptable values in sensitivity, response time, and lifetime in theirinitial proof-of-concept demonstrations. In addition to the aboveapproaches, there exists relatively mature optical-based (LED) systemsfor O₂ sensing, manufactured at Ocean Optics (see ref. xiv). Theunderlying principle in this case is based on the quenching ofoxygen-sensitive fluorescent dyes, such as Ruthenium (Rt) or porphyrin,embedded in thin films. A step forward is now offered by OLEDs, built atISTI/Ames Labs and also developed at Tokai University, which areextremely appealing, given that the lightsource/sensor/controls/detector are integrated into a singleminiaturized chip. In these last two examples, the devices are aimed atonly one particular molecule. The device applicability and performancelimitations of these sensors, which are, at present, still in thedevelopment phase, are principally functions of the selection andinteraction dynamics of the given surface coatings with thetrace-gas(es) of interest.

TABLE 1A (Prior art) Measured Gas Detection Response Cross-Configuration Parameter Specie Range Times sensitivity 0.25 m hollow IRabs/NIR NO₂/CO, 10-200 ppm 0.02-7 s yes fiber/microbore abs C₂H₂ FTIR,20 cm wdg IR abs CO₂ 10-200 ppm yes 9 m fiber QC IR absFina NO  0.06 ppmyes laser Pd MOS ΔC/C H₂ 0.2-100% 75 ms-20 s yes capacitors & ΔR/RHEMT/Schottky diodes Galvanic/Electro Δl/l by ox- O_(x), CO_(x)NO_(x) .. .  0-999 ppm 5-30 s no chemical cells red ox (cm³) Porous Si Δl/l/PLNO_(x)   0.1-2 ppm 10 s m (PL yes FET/Thin film recovery) Porous metal-ΔR/R NO₂, CO_(x)  0.05-3 ppm 1-15 m Partial oxides, metal- (adsorp.(high T, semic. IDT rates) noble electrode metals) LED or OLED Dyefluorescence O₂  0-40 ppm 1 μs-1 s yes w/Dye-Thin quenching film (100μm²) fiber to spectrum He—Ne, substr. Wavelength H2NO3   ~100 ppm 30-60m yes w/Ag nano Raman shift sarin, HE clusters (100 μm²), Δλ RamanSpectrograph White Light ΔR/R/ H₂ 0.2-4%/ 5-130 s/~10 s yes PdAu thinfilm ΔT/T ~10 s reflectance/Pd nanotapered fiber (μm² × mm)λ = 1.5 μm

TABLE 1B (Prior art) Operational Configuration Lifetime Point ReferencesShortfalls 0.25 m hollow X Standard Saito 1992 Dimensionsfiber/microbore (fouling) Prickell 2004 Occlusion Diffusion FTIR, 20 cmwdg X Standard Kozodoy 1996 Dimensions (fouling) Occlusion Diffusion 9 mfiber QC X Standard Fetzer 2003 Dimensions laser (fouling) OcclusionDiffusion Pd MOS Few T&RH Keibali Corp Cross-sensitivity capacitors &years control H2scan-SNLA Irreversibility HEMT/Schottky (MEMS) TimeResponse diodes Lifetime Galvanic/Electro Few T&RH BW Tectn/RKICross-sensitivity chemical cells mos-few (MEMS) Instrum Irreversibility(cm³) years Time Response Lifetime Porous Si Limited StandardSberveflieri 2006 Cross-sensitivity FET/Thin film (PL red Sailor 1996Irreversibility ox) Time Response Lifetime Porous metal- Depend Tcontrol: Barrettino 2006, Cross-sensitivity oxides, metal- on uhotplateKim 2005 Irreversibility semic. IDT system (cm³) Wang 2003, TimeResponse electrode futurlec.com Lifetime LED or OLED Few yrs Std/T&HROcean Contenders w/Dye-Thin control Optics/ISTI_Ames Equipment film (100μm²) (MEMS) Labs/Tokai Un. Lifetime fiber to spectrum Coatingdevelopment He—Ne, substr. X (oxid) Standard Stokes 2000, 2005Contenders w/Ag nano Talley 2004 Equipment clusters (100 μm²), Yan 2005Lifetime Raman Coating development Spectrograph White Light >1 yrStandard + Zhao 2005 Contenders PdAu thin film T&RH Villatoro 2005Equipment reflectance/Pd control Olpiski 2004 Lifetime nanotaperedCoating development fiber (μm² × mm) λ = 1.5 μm

SUMMARY OF THE INVENTION

Embodiments of the present invention provide unique configurations forin-situ gas detection, and include miniaturized photonic devices,low-optical-loss, guided-wave structures and state-selective adsorptioncoatings. High quality factor (Q=λ/dλ>10⁵ to 10¹⁰) semiconductorresonators have been demonstrated in different configurations, such asmicro-disks, micro-rings, micro-toroids, and photonic crystals with theproperties of very narrow NIR transmission bands and sensitivity up to10⁻⁹ (change in complex refractive index) (see ref. xv). The devices aretherefore highly sensitive to changes in optical properties to thedevice parameters (e.g., refractive index and absorption) and can betunable to the absorption of the chemical species of interest. Moreover,appropriate coatings (functionalization) applied to the device enhancestate-specific molecular detection.

Additional benefits of this class of sensor include optical fibercompatibility (e.g., remote addressing and probing), high-density devicepacking, integrability with sources and/or detectors, and enhanced S/Nlevels using differential techniques, such as common-mode rejection.Fabrication tolerances, optical properties of sensing materials,lifetime and ruggedness constitute other relevant performance metricsthat are addressed by this invention (see ref. xvi).

Embodiments of the present invention provide for compact, robust sensorsbased upon adsorption spectroscopy and integrated transductionmechanisms. This class of sensor is referred to herein as a resonantoptical transducer (ROT). The sensors function via changes in theiroptical properties in response to the state-selective adsorption of adesired species. Special coatings, deposited at critical locationswithin the sensor structure, provide necessary media for the desiredstate-selective, trace-gas adsorption as well as for discrimination inthe presence of non-critical species. Examples of ROT embodimentsinclude specially coated edge emitting lasers (EELs), vertical cavitysurface optical amplifiers (VCSOAs) and vertical cavity surface emittinglasers (VCSELs). The extension of a 1D sensor to a 2D platform resultsin an array of functionalized and tunable cavities, with application tomultiplexed trace-gas detection and discrimination against false alarms.Tables 1C and 1D provide data for embodiments of the present inventionfor comparison with the prior art shown in Tables 1A and 1Brespectively.

TABLE 1C Con- Measured Gas Detection Response Cross- figurationParameter Specie Range Times sensitivity TOCs & IR abs CO_(x),, 1-100ppm Ns-μs yes ROTs ΔP/P or NO_(x),, (equipment Δλ/λ CH₄, HCI, limitedHF, O₂, H₂

FIG. 1D Operational Configuration Lifetime point References ShortfallsTOCs & N/A Standard + N/A New ROTs T&RH Technology Control development

This class of resonant optical transducer, when coated withfunctionalized films engineered to adsorb specific gas species, canfunction, e.g., when gases without a significant near infrared (NIR)signature are to be detected. That is, in situations where there isminimal spectroscopic optical interaction cross section with the desiredspecies, the use of adsorption mechanisms can be utilized to advantage.In these cases, the adsorbed species can modify or perturb specificoptical properties of a resonant cavity, which can be subsequentlydetected and sensed via non-resonant optical probes. That is, theoptical probe beam does not interact directly with the molecularspecies, but, instead, probes optical perturbations of the structureresulting from the presence of the given species to be sensed.Therefore, the specific wavelength of the optical sensor is notconstrained to a specific optical absorption feature of the molecule.

Hence, as the desired species is adsorbed onto a state-selective coatingwithin the optical sensor, the coating optical properties are modifiedby the change in the phase (refractive index) and/or absorption (opticalloss) of light as it interacts with the now-modified surface coating.The appropriate use of high-gain optical amplifying media and/or laserscan result in trace-gas sensors with high sensitivity and responsivity,owing to the highly nonlinear dependence of these structures to minutephase and/or amplitude perturbations at or near an oscillationthreshold. Resonant cavities with high quality factor (Q), oramplification, can enhance the sensitivity of the devices. The operationwavelength of the devices is, in this case, independent of the gas andonly a very small footprint is necessary. Nevertheless, a tunable sourceprovides several advantages over a fixed-wavelength optical probe beam,especially in the presence of systematic errors.

Regarding the probe beam, an optical source with a 30-50 nm wavelengthtuning range is beneficial to track the resonance of the opticalamplifier when the trace-gas molecules infiltrate or bond to thespecific coating and change the resonance condition by effectivelychanging the optical path. In addition to monitoring peak-amplitudevariations of the fixed, initial cavity resonance, source tunability cancircumvent various systematic effects, as well as determine the optimalwavelength overlap of the probe beam with the Fabry-Perot modes of theoptical cavity. By analogy, in the case of VCSELs, tunability providessimilar benefits, in addition to molecular detection via resonant photoninteractions, as discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the disclosure, illustrate embodiments of the invention and, togetherwith the description, serve to explain the principles of the invention.

FIG. 1 depicts an embodiment using an edge-emitting laser (EEL), withlateral functional coatings as a trace-gas sensor.

FIG. 2 shows results of a simulation depicting the optical mode profileof an EEL output beam.

FIG. 3 shows results of a measurement of fractional changes in theoptical reflection and transmission coefficients of a 11.83 nm thin filmof Pt in the presence of N₂ and H₂ gases.

FIG. 4 shows simulated H₂ trace-gas sensor responses for a Pd-coatedpassive waveguide (WG), a single-section EEL, and a multi-section EEL.

FIG. 5 depicts an embodiment using a vertical cavity surface opticalamplifier (VCSOA) with an additional oxide layer in the mirror stack todetect adsorbed gases.

FIG. 6 depicts an embodiment using a vertical cavity surface emittinglaser (VCSEL) with an additional oxide layer in the mirror stack todetect adsorbed gases.

FIG. 7 shows results of the output optical spectrum of a VCSEL asfunction of voltage bias applied to a longitudinally moveable, laseroutput mirror.

FIG. 8A shows a plot of a voltage waveform, as applied to a membranemirror to evaluate the transient response of a tunable VCSEL.

FIG. 8B shows a measurement of the transient velocity response of aVCSEL membrane mirror in the presence of the applied drive waveformshown in FIG. 8A.

FIG. 8C shows a measurement of the transient displacement response of aVCSEL membrane mirror in the presence of the applied drive waveformshown in FIG. 8A.

FIG. 9 depicts a system-level embodiment of a trace-gas sensor 2D-arrayusing an optical fiber as a remote optical interrogation probe.

FIG. 10 depicts a system-level embodiment of a trace-gas sensor arrayusing a 2D CMOS array for parallel sensor control and readout.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of a present ROT is shown in FIG. 1, the basic structureof which is a semiconductor-based edge-emitting laser, EEL 200. Thebasic EEL, which is well known in the art, is augmented with astate-selective thin-film functionalized adsorption layer, therebyresulting in the trace-gas sensor. The EEL is comprised of a substrate,210, upper (gain) guiding ridge layer 220, and top and bottom contacts,230 and 240, respectively, for biasing. In some cases, the top contactcan be partitioned, so that different regions within the laser canpossess differing amounts of optical gain or loss. The basic guided-waveoptical mode, 250, is confined vertically by the basic waveguidestructure and laterally, via gain guiding. The EEL optical cavityconsists of a pair of Fresnel reflective air/semiconductor interfaces,255, formed at the front and rear substrate facets, respectively, normalto the optical axis. The semiconductor materials and structure of thebasic laser are well known in the art. As an example, the EEL laser canbe comprised of Group III-V materials, e.g., InAs, InP and GaAs, and itsternary and quaternary alloys (e.g., InGaAs and InGaAsP, respectively);and the gain medium can be configured in a variety of architectures, anexample of which is a double quantum-well structure (DQW) using similarmaterials.

In this embodiment, a set of functionalized surface regions, 260 and270, respectively, are each coated with a suitable state-selectiveadsorptive thin-film layer. These coatings are deposited on the uppersurface of the EEL, and positioned on either side of the ridge guidinglayer, 220. By changing the coating, different gases can be detectedwith a well-defined selectivity and specificity as a function of thecoating material. During operation, as the desired species is adsorbedby the thin film, the optical properties of the material will bemodified, such as its refractive index and/or absorption. Thesetrace-quantity induced thin-film optical perturbations will, in turn,affect the output of the EEL, in terms of its laser output power, outputspectrum and modal characteristics. The sensitivity to the adsorbedmaterial is enhanced by the highly nonlinear dependence of the laserproperties to the optical cavity characteristics. In essence, theinternal laser photons traverse the optical cavity multiple times (100sto 1,000s of traversals, depending on the specific laser design),thereby probing the modified cavity parameters in a like manner, and,thusly, effectively amplifying the sensitivity of the sensor. Variousregions on the coatings can be masked via optional coverings, 265, asmay be deemed necessary for a given structure.

An example of a coating, with application to the detection of tracequantities of H₂ gas, is a thin film of either Pd or Pt. The optical andelectrical properties of these materials change in the presence of H₂which can be detected even in a background of atmospheres of N₂. In aseparate set of experiments, ellipsometric measurements were performedon Pd and Pt thin films to quantify the complex index of refraction atvarious optical wavelengths of interest, with and without the presenceof H₂. These measurements enable more accurate quantitative informationto be obtained using the thin-film coated trace-gas sensors, discussedherein. In addition, they provide more accurate data as input to thesensor simulations so that their performance can be optimized.

FIG. 2 shows data of the measured fractional change in reflection (FCR),ΔR/R₀, and transmission (FCT), ΔT/T₀ for an optically thin Pt 11.83 nmfilm, in the presence of H₂. The fractional change technique isbeneficial because it normalizes out collection efficiencyproportionality factors. The steady state FCR and FCT values for 1% H₂were 2.06% and −2.32%, respectively. A Pd 26.94 nm film had a similarresponse with FCR=1.33% and FCT=−1.42%. The anomalous reflectanceincrease in optically thin films can be attributed to reflections frommultiple interfaces. For the thick films, statistically significantchanges were not observed.

The Pt and Pd thin-film rise times were quite different; 10 minutes and10 seconds, respectively. After the H₂ is shut off, the reflection andtransmission for both films begin moving away from the baseline. Thisbehavior is under investigation. With the measured 1550 nm values for nand κ for each film, the spectrally averaged FCR and FCT were simulatedfor Δn=0.001 and Δκ=0.001 using the transmission matrix technique.Assuming linear dependencies, the combination of Δn and Δκ that yieldthe measured FCR and FCT can be solved algebraically from a 2×2 matrix.Estimates of Δn and Δκ made this way were very close to the solutionsobtained from iterative searching. For 1% H₂ in N₂, we find Δn=−0.089and Δκ=0.181 for the Pt 11.8 3 nm film, compared to Δn=−0.033 andΔκ=0.067 for the Pd 26.94 nm film. The increase in extinctioncoefficient is unexpected given previous discussions and trends for Pdfilms. Additional experiments are needed to confirm this trend and theparameter extraction technique. Nonetheless, these measurements provideuseful input to the modeling and simulation of the H₂ trace-gas sensors.

The performance of the embodiment shown in FIG. 1 was modeled using anumerical simulation code. As a specific example of an EEL sensor, alaser with a graded-index separate confinement heterostructure (GRINSCH)configuration was considered, using a standard double quantum well (DQW)structure. Lasers without lateral surface coatings have been previouslyfabricated and thoroughly characterized and modeled. For this choice ofmaterial and geometry, the laser wavelength varies from 950-970 nm,depending on bias and temperature. The EEL geometry that was modeled isa multiple section 2 μm×250 μm (EEL) with a 50 nm thick Pd surfacecoating that is located 100 nm above the active region and extends 3 μmlaterally on either side. Thus, the total sensor width is 8 μm (see FIG.1 for labeling of the dimensions). The optical mode has a small overlap,Γ_(Pd)≈1×10⁻⁴, with this thin coating. H₂ gas reacts with Pd to form PdHthereby reducing the internal loss seen by the laser mode, α_(i), from58 cm⁻¹ at a rate of 0.03 cm⁻¹ per 100 ppm of H₂ for our example sensor.Thus, the laser's output power increases rapidly in response to theamount of adsorbed H₂.

FIG. 3 shows results of a simulation to model the laser sensor. A 1-Drate equation model, previously verified against experimental L-I data,was used to calculate the response for L=250 μm, W=2 μm, V=100 nm, andt=50 nm for three types of Pd-coated sensors: a passive waveguide (WG),a single section EEL, and a multiple section EEL, the latter of whichincludes a 37.5 μm long unbiased saturable absorber with ≈15% opticalabsorption. (FIG. 1 shows the dimensional labeling.) For a faircomparison, we adjust the biases so that the sensor's output opticalpower is fixed at 10 μW for zero H₂. It is also assumed that laseroutput power measurement has an instrument uncertainty of ≈10⁻³.

For this set of parameters, the predicted limits of detection (LODs) forthe three structures were 138 ppm, 4 ppm, and 1 ppm, respectively. Thepassive WG requires 43□W of input optical power, whereas the EELs needno input optical power but use 8 mW and 13 mW, respectively, ofelectrical power, given an operation voltage of 1.5 V. Shorter cavitylengths used less power but had higher minimum detection limits (MDLs).The resonant cavity of the EEL provides a strong nonlinearity in thesensor response, i.e., the lasing knee, and thereby reduced the LODcompared to the passive WG. The saturable absorber further amplifiesthis lasing knee nonlinearity according to the gain-lever effect.

The simulation also provided spatial information as to the guided-wavemode supported by the EEL structure. FIG. 4 shows a typical “pearshaped” spatial mode profile for the case W=2 □m and t_(Pd)=50 nm andthe inset shows t_(Pd)=0 nm. Both have elliptical central fieldcontours, but the contours at the field tails for t_(Pd)=50 nm aresignificantly squeezed in by the Pd coating, despite the very smallmodal overlap. The high sensitivity of the spatial-mode dependence on Pdcoating is, most likely, due to the optical cavity losses experienced bythe guided-mode field in the presence of the thin film. Given thedependence of the mode profile as a function of the adsorbing coating, areal-time measurement of changes in the spatial-mode pattern may, infact, provide additional evidence regarding the presence of trace-gasspecies.

These chip-scale EEL-based sensors offer high sensitivity, wide dynamicrange, inline integration with photodetectors and 2-D scalability fordrift compensation and for detection and identification of multiplespecies. For maximal sensitivity, a large spatial-mode field overlapwith the coating is needed. This can be achieved with a narrow ridge, athick coating layer, zero horizontal distance between ridge and coating,and a small vertical separation between coating and active region. Forthe 8 μm×250 μm H₂ sensor with W=2 μm, V=100 nm, t=50 nm, a LOD of 1 ppmis predicted.

Returning to FIG. 1, the absorption layer(s) can be deposited at otherlocations on the EEL and in various patterns, with the proviso that theoptical parameters of the EEL output and/or the cavity mode pattern,250, is measurably altered as a result of the adsorption of the desiredgas specie(s). The EELs can be configured using differently designedoptical resonators in place of (broadband) Fresnel reflectiveinterfaces. Examples of different EEL resonators include those formedusing distributed Bragg reflecting (DBR) gratings as narrowband endmirrors or, distributed Bragg feedback grating structures (DFB), as wellas micro-toroidal and ring resonators. The DBR and DFB lasers canprovide single longitudinal-mode operation, which may increase thesensitivity of the resultant EEL gas sensor, as well as provide forenhanced performance detection via common-mode rejection techniques(using closely situated pairs of coated and uncoated EELs, on a commonsubstrate).

A laser with a suitable output wavelength can be designed so that thequantum levels of the desired species (or, isotope, thereof) to besensed can be resonant with the optical mode. In this case, a single EELcan provide a sensor with a dual-detection modality: (1) simultaneousresonant optical detection of the desired trace species via directoptical interaction of the molecule of choice with the laser photons and(2) broadband detection of the trace species via state-selectiveadsorption of the molecule of choice, as manifested by non-resonantchanges in the laser output characteristics, such as cavity mode,wavelength, polarization, etc.

FIG. 5 shows an embodiment of a trace-gas sensor using a speciallycoated vertical cavity surface optical amplifier (VCSOA), 600. The basicVCSOA is similar in its configuration to that of a conventional verticalcavity surface emitting laser (VCSEL), except for the fact that theVCSOA is not an oscillator, but, instead, a multi-pass opticalamplifier. In essence, the VCSOA is a VCSEL that operates below thelaser threshold condition. Typically, the VCSOA is fabricated usingeither a lower gain and/or a lower mirror reflectivity relative to aVCSEL for a given bias.

The VCSOA and the VCSEL devices are well known in the art. In general,both devices are comprised of a substrate 610, with an internallyfabricated single or multi-layer reflecting mirror, 620, whosereflectivity is 100%. An internal gain medium, 630, typically asemiconductor based multi-layer structure, is grown onto the mirror 620.In most cases, an input/output mirror, in the form of a partiallytransmitting mirror, 640, is subsequently grown above the gain medium,completing the basic structure (for simplicity, electrical contactinglayers and various buffer layers are not described, as they are wellknown in the art).

During operation, an optical interrogation beam, 650, is incident uponthe VCSOA. The incident beam interacts with the optical cavity,resulting in an amplified output beam, 660. Owing to the geometry of thedevice, the counter-propagating input and output beams, 655, overlap inspace. Typically, an optical circulator, 670, is employed to distinguishthe input and output beams.

In the present embodiment, the basic VCSOA is augmented through the useof a vertically displaced, deformable membrane, 680, typically, SiN_(x)with a coating of Al, upon which the input/output coupling mirror, 640,is grown. One function of the deformable membrane/mirror is to vary theoptical cavity length, and, therefore, to control the spectralproperties of the VCSOA. For a given interrogation wavelength, thecavity can therefore be tuned so that the Fabry Perot mode(s) within thecavity are in resonance with the input probe beam for maximumsensitivity of the sensor.

The presence of the vertically displaced output mirror, 640, results inan air gap, 690, formed between the upper surface of the basic structureand the inner-cavity surface of the output mirror. During operation, thegas sample to be analyzed is allowed to flow into the intra-cavity gap,690.

The trace-gas sensor also consists of a metal oxide coating layer, 695,grown onto the inner-cavity surface of the membrane (680)-mirror (640)structure. This oxide layer is selected to modify the effectivecoupling-mirror reflectivity in the presence of the trace-species to besensed. The change in reflectivity is due to a change in the refractiveindex and/or optical absorption in the coating layer as the trace-gas isadsorbed in the film. This change in the reflectivity, as a result ofthe presence of the trace-gas, can be detected by measuring changes inthe VCSOA output beam, 660, including changes in the optical powerand/or shifts in the output spectrum. Examples of adsorbing materialsand coatings include WO₃, SnO₂, PdO, ZnO, and porous Si. The opticalproperties of these coatings are modified in the presence of a varietyof trace gases, including NO_(x), CO, H₂S and Cl₂. By changing thecoating, different gases can be detected, with varying degrees ofselectivity and specificity.

Turning now to FIG. 6, an embodiment using a VCSEL, 700, is shown, thatcombines two different, yet, complementary, trace-gas detectionmodalities into a single device: (1) sensing changes in the propertiesof an optical device via non-resonant adsorption of a given specieswithin the cavity and (2) sensing resonant photon interactions of theintracavity optical beam directly with a specific molecular absorptionfeature of the trace-gas species. The first modality has been describedabove, whereby indirect optical measurements are performed to infer thepresence of a given adsorbed species in an optical resonator. The secondmodality, which complements the first approach, is a form of NIRabsorption spectroscopy performed within a laser cavity.

The basic configuration, as shown in FIG. 6, is that of a VCSEL, with avertically displaced, membrane-mounted output mirror, 740, anintra-cavity air gap, 790, and an internal adsorption oxide layer, 795.Hence, the basic configuration is similar to that of the VCSOAembodiment (see, e.g., FIG. 5). As discussed above, the VCSEL is anoptical oscillator (a laser), and as such, generates an output opticalbeam, 760. Therefore, no external interrogation beam is required, as theoptical oscillator provides an effective probe beam within the cavity.Since the oscillating beam passes through the air gap multiple times,the presence of a trace gas can be sensed via changes in the outputbeam, as before. However, the output beam changes can be induced byeither cavity perturbations resulting from changes in the internaloptical coating, 795, and/or by direct optical absorption with the gassample itself. The presence of a trace-gas can therefore be inferred bychanges in the VCSEL laser output beam, including its output power andits output spectrum. Furthermore, the VCSEL operating wavelength can betuned via changes in the vertical displacement of the moveable mirror.Thus, the VCSEL trace-gas sensor can be described as a wavelengthtunable, multi-pass cell with optical gain.

In operation, the laser is electrically driven above threshold, with thegas sample allowed to flow through the air gap, 790. The presence of thedesired trace-gas spoils the gain-loss balance necessary for lasing byincreasing the absorption losses within the cavity, either by resonantcoupling with the gas molecule within the air gap and/or by non-resonantoptical cavity changes induced by adsorption of the gas into theinternal oxide layer, 795. In either case, the high-Q of the VCSELstructure enhances the sensing by adsorption and/or absorption, as thelight is reflected multiple (>100) times within the resonant cavity,formed on one end by the external membrane distributed Bragg reflecting(DBR) mirror, 740, and on the other end by the internal distributed DBRmirror, 720. During operation, the lasing power and/or output spectrumcan be monitored remotely by transmission through an optical fiber ordirectly by an integrated detector.

A VCSEL with a moveable membrane-based output coupler was fabricated andevaluated in terms of its spectral properties as well as its temporalresponse to transient changes in the optical cavity length. FIG. 7 showsresults of the measured dependence of the VCSEL output spectrum fordifferent values of a bias control voltage, as applied to the moveablemirror membrane. As the membrane is electrostatically displaced, theoutput spectrum is observed to change correspondingly, shifting toshorter wavelengths as the cavity length decreases (increasing biasvoltages). The multiple spectral features for each measured spectralscan are attributed to the presence of the VCSEL's multiple longitudinalmodes, given the cavity design parameters. Figure A8-8C show results ofthe temporal response time of the moveable membrane. FIG. 8A shows thetransient control voltage, as applied to the membrane. The response ofthe membrane in the presence of this transient signal is shown in FIGS.8B and 8C, which, respectively, display measurements of the membranevelocity and displacement. The membrane response time is observed to bein the range of ≈10 μsec under these conditions.

FIGS. 9 and 10 show two different systems implementations of thetrace-gas sensor device, 1000 and 1100, respectively. In eachembodiment, a 2D array of resonant optical transducers is depicted inthe figures, as indicated by 1010, and 1110, respectively. In FIG. 9, aremote optical fiber interrogation approach is shown. In thisembodiment, the sensor package, 1010, can be placed at one location, andthe optical sources and/or detector components, 1050, can be locatedelsewhere, using an optical fiber, 1060, as an optical link. In FIG. 10,a compact 2D sensor/readout approach is shown, using CMOS technology forthe sensor control functions, 1170, and, additionally for 2D parallelreadout and pre-processing functions, 1180. The geometry in the lattercase is symmetrical, in that the sensor and readout arrays areidentically configured, forming a 2D array of free-space opticalinterconnects linking the sensor and the interrogator arrays.

In both systems implementations, each 2D array is partitioned into threeregions, 1020, 1030 and 1040 in the case of FIG. 9 and 1120, 1130 and1140 in the case of FIG. 10. Each of the three respective regionsconsists of sensors designed to respond to a unique trace species. Thepresence of multiple sensors for each species provides redundancy interms of graceful degradation, as well as the potential to provide forminimal false alarm rates, systematic errors, etc.

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The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments disclosed were meant only to explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

We claim:
 1. A resonant optical transducer, comprising: a firstelectrical contact layer; a substrate in operative contact with saidfirst electrical contact layer, said substrate including an internallyfabricated mirror; an active gain medium in operative contact with saidsubstrate; a second electrical contact layer in operative contact withsaid gain medium; two pillars in operative contact with said secondelectrical contact layer; a top layer supported by said pillars, whereinsaid top layer together with said second electrical contact layer andsaid pillars, forms an optical cavity having an intra-cavity gap,wherein said top layer includes a state selective oxide layer inoperative contact with a reflective layer; and a mirror in operativecontact with said top layer.
 2. The resonant optical transducer of claim1, further comprises a deformable membrane layer in operative contactwith and located between said state selective oxide layer and saidreflective layer.
 3. The resonant optical transducer of claim 1, whereinsaid first electrical contact layer comprises a first dopant, the ROTfurther comprises a doped layer between said second electrical contactlayer and said gain medium, wherein said doped layer comprises a seconddopant, wherein one of said first dopant or said second dopant comprisesan n material and the other comprises a p+ material.
 4. The resonantoptical transducer of claim 1, wherein each pillar of said two pillarscomprises electrically insulating material.
 5. The resonant opticaltransducer of claim 4, wherein said electrically insulating materialcomprises a dielectric material.
 6. The resonant optical transducer ofclaim 1, wherein said first electrical contact layer is an n-contactlayer.
 7. The resonant optical transducer of claim 6, wherein saidn-contact layer comprises Ge/Au/Ni/Au.
 8. The resonant opticaltransducer of claim 1, wherein said substrate is an n-doped GaAssubstrate.
 9. The resonant optical transducer of claim 1, wherein saidinternally fabricated mirror is a multi-layer mirror.
 10. The resonantoptical transducer of claim 9, wherein said multi-layer mirror is a 40.5period AlGaAs distributed Bragg reflector.
 11. The resonant opticaltransducer of claim 10, wherein said DBR comprises a reflectivity of≈100%.
 12. The resonant optical transducer of claim 1, wherein saidactive gain medium comprises a semiconductor based multi-layerstructure.
 13. The resonant optical transducer of claim 12, wherein saidsemiconductor based multi-layer structure is selected from the groupconsisting of an AlGaAs quantum well, InGaAs and InGaAsSb.
 14. Theresonant optical transducer of claim 1, wherein said second electricalcontact layer comprises p+ GaAs.
 15. The resonant optical transducer ofclaim 1, wherein said state selective oxide layer comprises metal oxide.16. The resonant optical transducer of claim 1, wherein said deformablemembrane comprises SiN_(x).
 17. The resonant optical transducer of claim1, wherein said reflective layer comprises aluminum.
 18. The resonantoptical transducer of claim 1, wherein said mirror comprises a TiO₂/SiO₂distributed Bragg reflector pillar.
 19. The resonant optical transducerof claim 1, wherein said state selective oxide layer comprises oxidematerial selected from the group consisting of WO₃, SnO₂, PdO, ZnO andporous Si.
 20. The resonant optical transducer of claim 1, wherein saidROT comprises at least one opening to allow gas to enter said opticalcavity.
 21. A method, comprising: providing the resonant opticaltransducer (ROT) of claim 1; operating said ROT below its laserthreshold condition; coupling an optical interrogation beam into saidoptical cavity such that said beam oscillates therein and an amplifiedoutput beam is produced; allowing a gas sample to flow into saidintra-cavity gap, wherein said state selective oxide layer comprisesoxide material selected to modify the effective coupling-mirrorreflectivity of said reflective layer in the presence of a trace-speciesto be sensed; and measuring changes in said output beam to determine ifsaid trace-species is present within said optical cavity.
 22. The methodof claim 21, wherein said changes are relative to said opticalinterrogation beam.
 23. The method of claim 21, wherein said changesinclude changes in the optical power and/or shifts in the spectrum ofsaid amplified output beam relative to said optical interrogation beam.24. The method of claim 21, wherein said effective coupling-mirrorreflectivity is modified due to a change in the refractive index and/oroptical absorption in the coating layer as the trace-gas is adsorbed insaid oxide layer.
 25. The method of claim 21, wherein said ROT furthercomprises a deformable mirror between said oxide layer and saidreflective layer, the method further comprising altering the shape ofsaid deformable mirror to tune said optical cavity.
 26. The method ofclaim 25, comprising tuning said deformable mirror so that at least oneFabry Perot mode within said optical cavity is in resonance with saidinterrogation beam.
 27. A method, comprising: providing the resonantoptical transducer (ROT) of claim 1; operating said ROT at or above itslaser threshold condition, wherein a intra-cavity beam is producedwithin said optical cavity and oscillates therein and wherein an outputbeam is coupled out of said optical cavity; allowing a gas sample toflow into said intra-cavity gap, wherein said state selective oxidelayer comprises oxide material selected to modify the effectivecoupling-mirror reflectivity of said reflective layer in the presence ofa trace-species to be sensed; and measuring changes in said beam todetermine if said trace-species is present within said optical cavity.28. The method of claim 27, wherein said changes are relative to saidoptical interrogation beam.
 29. The method of claim 27, wherein saidchanges are selected from the group consisting of (i) changes in theproperties of said oxide layer via non-resonant adsorption of a givenspecies within said cavity and (ii) resonant photon interactions of saidintra-cavity optical beam directly with a specific molecular absorptionfeature of a trace-gas specie.
 30. The method of claim 27, wherein saideffective coupling-mirror reflectivity is modified due to a change inthe refractive index and/or optical absorption in the coating layer asthe trace-gas is adsorbed in said oxide layer.
 31. The method of claim27, wherein said ROT further comprises a deformable mirror between saidoxide layer and said reflective layer, the method further comprisingaltering the shape of said deformable mirror to tune said opticalcavity.
 32. The method of claim 31, comprising tuning said deformablemirror to provide at least one Fabry Perot mode in said optical cavityin resonance with said intra-cavity beam.